202

Sir I s A A c Newton’s Book II.

great, as the addition to the earth’s adtion by the fun in thequarters a ; fo that upon the whole, the power of the earthupon the moon is diminifhed by the fun, and therefore ismoft diminifhed, when the adtion of the fun is ftrongeft: butas the earth by its approach to the fun has its influence leffen-ed, the moon being lefs attracted will gradually recede fromthe earth ; and as the earth in its recefs from the fun recoversby degrees its former power, the orbit of the moon muff a-gain contract. Two confequences follow from hence : themoon will be moft remote from the earth, when the earth isneareft the fun; and alfo will take up a longer time in per-forming its revolution through the dilated orbit, than throughthe more contracted.

21 . These irregularities the fun would produce in the'moon, if the moon, without being adted on unequally by thefun, would defcribe a perfedt circle about the earth, and inthe plane of the earth’s motion ; but though neither of thelefuppofitions obtain in the motion of the moon, yet the fore-mentioned inequalities will take place, only with fome diffe-rence in refpedt to the degree of them ; but the moon by notmoving in this manner is fubjedt to fome other inequalities al-fo. For as the moon defcribes, inftead of a circle concentri-cal to the earth, an ellipfis, with the earth in one focus, thatellipfis will be fubjedted to various changes. It can neitherpreferve conftantly the fame pofttion, nor yet the fame fi-gure ; and becaufe the plane of this ellipfis is not the fame

* Newton. Princ. Lib.I. prop. 66 . coroll, 7.

with